1. Field of the Invention
The present invention generally relates liquid crystal display devices (LCDs) and, more particularly, is directed to an active matrix type liquid crystal display device in which a plurality of pixels, each having a switching transistor are aligned in a matrix form.
2. Description of the Prior Art
The structure of one pixel of a conventional active matrix type liquid crystal display device is shown in FIGS. 1 and 2. In FIG. 1, reference numeral 1 designates a transparent pixel electrode constituting a pixel (a liquid crystal cell (LC)), and 2 a switching thin film transistor which drives the pixel. A selecting line 3 for selecting a row of the respective pixels (LC) is arranged between respective rows of the pixel electrodes 1, while a signal line 4 for supplying a video signal is arranged between respective columns of the pixel electrodes 1. Then, a drain 5D of the thin film transistor 2 is connected to the pixel electrode 1, its source 5S is connected to the signal line 4, and its gate 5G is connected to the signal line 3 (in FIG. 1, the gate 5G and the signal line 3 are used in common).
The specific structure thereof is as shown in FIG. 2, wherein a polycrystalline silicon layer 8 constituting the thin film transistor 2 is deposited at a required location on an insulating substrate 7 made of glass or the like, and on this polycrystalline silicon layer 8 there is formed the gate electrode 5G (or the selecting line 3) made of, for example, an impurity doped polycrystalline silicon through a gate insulating layer 9. A first insulating layer 10 made of, for example, a PSG (phospho-silicate glass) layer is formed over the entire surface of the gate electrode 5G so as to cover the same, and through a contact hole 12 formed through the first insulating layer 10 the signal line 4 made of, for example, Al is connected to the source 5S. Further, a second insulating layer 11 made of, for example, a PSG layer is deposited on the entire surface. Through, the first and second insulating layers 10 and 11, there is formed a common contact hole 13 at a position corresponding to the drain 5D, and then a transparent conductive layer made of, for example, an ITO (indium tin oxide) layer is formed on the entire surface including the contact hole 13. A patterning is performed therefor to form the transparent pixel electrode 1 which is connected to the drain 5D through the contact hole 13. Then, another insulating substrate 14 made of glass or the like which has a light shielding layer 15 formed in its inner surface at positions corresponding to a wiring portion (a portion where the selecting line 3, the signal line 4 and so on exist) and the thin film transistor 2 and an opposite electrode 16 formed on its entire surface including the light shielding layer 15 is disposed in opposition to the insulating substrate 7. A liquid crystal layer 17 is then enclosed between both the substrates 7 and 14 to thereby constitute a liquid crystal display device 18.
FIG. 3 shows another example of the convention liquid crystal display device. In this example, the thin transistor 2 formed of the polycrystalline silicon layer 8, the gate insulating layer 9 and the gate electrode 5G (i.e. the selecting line 3) is formed on the insulating substrate 7 on which the insulating layer 10 is formed. In the next process, the transparent pixel electrode 1 is formed by the ITO layer so as to be connected to the drain 5D of the thin film transistor 2 via the contact hole 13 of the insulating layer 10, and the signal line 4 made of, for example, Al is connected to the source 5S of the thin film transistor 2 via the contact hole 12 of the insulating layer 10.
Further, in the liquid crystal display device, in order to improve the pixels, additional capacitance i.e. storage capacitance C.sub.S must be added to every pixel LC, and it is preferable that the value of the storage capacitance C.sub.S is as large as possible in order to prevent the occurrence of flicker. To this end, as, for example, shown in FIG. 4, it is proposed that a polycrystalline silicon layer 8 is elongated from the source 5D and the storage capacitance C.sub.S is constructed between an elongated portion 8A of this polycrystalline silicon layer 8 and an electrode wiring 19 for exclusive use of the storage capacitance C.sub.S interconnected via an insulating layer (not shown).
One of the problems implied in the above-mentioned prior-art liquid crystal display device is the connection between the drain 5D of the thin film transistor 2 and the transparent pixel electrode 1. Due to the convenience for the manufacturing process, the polycrystalline silicon layer 8 of the thin film transistor 2 is formed in a lower portion, and the transparent pixel electrode 1 made of an ITO layer is formed on the topmost layer, whereby both are connected through the contact hole 13. However, the height h (FIG. 2) of this contact hole 13 has approximately 1 .mu.m, which makes it difficult to connect the ITO layer 1 having a thickness of approximately 0.1 .mu.m to 0.15 .mu.m (film thickness because of the transparent pixel electrode) and formed of ITO layer with a favorable coverage. Therefore, the contact hole 13 is formed in a taper shape to provide an easier coverage, however, it cannot be said that the situation is 100% satisfactory.
Further, if the contact hole is made smaller (e.g. 0.5 .mu.m to 1.0 .mu.m) in order to increase an opening ratio of the pixel, then it becomes difficult to interconnect the ITO layer 1 with an excellent coverage, and the contact resistance between the drain 5D and the ITO layer 1 is unavoidably increased by the amount corresponding to the reduced area of the contact hole. Particularly, since the ITO layer 1 contains oxygen, the silicon surface of the contact portion with the thin film transistor is naturally oxidized, thereby the ohmic contact being deteriorated. If the contact hole is made small, reliability of contact is considerably scattered, which results in the concentration of display pixel being scattered.
On the other hand, in a liquid crystal display device in which the storage capacitance C.sub.S is provided, if the electrode wiring 19 used exclusively for the storage capacitance C.sub.S is made of the same conductive material as that of the gate electrode 5G, that is, the selecting line 3, then the opening ratio of the pixel is sacrificed for the provision of the electrode wiring 19. Accordingly, the contact hole must be made as small as possible but if the contact hole is made small, the contact between the transparent pixel electrode and the ITO layer is lowered in reliability. Then, there arise similar problems mentioned above. A defective contact between the transparent pixel electrode 1 and the thin film transistor 2 appears as a point defect on the liquid crystal display device, resulting in the image quality being deteriorated.